Fluid refining method

ABSTRACT

A diffusion pump capable of separating impurities from its pump fluid during operation is disclosed. Skimming drains are provided in the pump&#39;s boiler for periodically skimming the evaporative surface of the working fluid. This eliminates nearly all contaminants of higher molecular weight than the pump fluid. In the foreline of the pump, a series of peripheral gutters are provided for trapping, separating and draining off condensates. The gutters facilitate the removal of impurities of lower molecular weight than that of the pump fluid. Means are also provided for further removing trace quantities of residual volatile impurities which tend to backstream up the diffusion pump barrel. The highly purified pump fluid allows for a more vigorously working evaporative surface, thereby increasing the throughput of the diffusion pump. Together with the elimination of volatile impurities from the pump barrel, this facilitates the attainment of significantly higher ultimate chamber vacuum. The withdrawn condensates and skimmed residues also form the basis for use of the apparatus as a device for obtaining a high degree of separation between liquids of very close vapor pressures.

This is a division of application Ser. No. 522,708 filed Nov. 11, 1974now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to diffusion pumps for producing high vacuum andmore particularly to an improved diffusion pump by which the pump fluidmay be separated from its impurities during operation.

Diffusion type pumps for producing high vacuum are well known. Theoriginal effort of Gaede around 1914 was supplemented by Langmuir in thedevelopment of the vertical jet diffusion pump disclosed in U.S. Pat.No. 1,393,350, and the single inverted jet or mushroom pump covered byU.S. Pat. No. 1,320,874. A multiple jet modification of the mushroompump is described in U.S. Pat. No. 1,367,865. Such pumps operate on theprinciple that a liquid having relatively heavy molecules is vaporizedin the pump by raising its temperature. The vapor comprising heavymolecules is directed by suitable nozzles in a direction away from theregion to be evacuated, towards a mechanical forepump. The acceleratedmolecules of vapor compress against molecules ahead of the nozzle,forcing them toward the mechanical forepump and thereby reducing thepressure within the evacuated region. The vapors are recondensed on acool wall of the pump where the liquid is permitted to return to thebottom of the pump to be reheated and vaporized.

Originally mercury was employed as the working fluid in these diffusionpumps, but later various organic oils and silicone fluids were developedas pump fluid, and today these fluids have almost completely replacedmercury in diffusion pumps. In particular, the silicone oils, of whichDC-705 (pentaphenyl trimethyl trisiloxane) manufactured by Dow CorningCorp., is an example, are in wide use today.

Performance of diffusion pumps has been erratic and subject to certainlimitations. It was long ago observed that the evaporative surface of astudied pump fluid in a pot still, when rapidly evaporating, seems toseparate into two different areas of turbulence, resulting in a"schizoid" evaporative surface. In one area of the surface, termed the"working" area, very rapid evaporation of the fluid takes place, whilein the other area, known as the "torpid" area, very little evaporationtakes place. A discussion of this phenomena is found in the article"Torpid Phenomena and Pump Oils", K. C. D. Hickman, The Journal ofVacuum Science and Technology, Volume 9, No. 2, and "Surface Behavior inthe Pot Still", K. C. D. Hickman, Industrial and Engineering Chemistry,Volume 44, No. 8. Since the torpid areas of the evaporative surfacewithin the diffusion pump boiler release vapors at a very low rate, thediffusion pump speed, throughput and ultimate vacuum attainable arelimited to the extent that the evaporative surface is affected bytorpidity.

Various remedies have been suggested to alleviate or overcome theproblem of torpidity in diffusion pump boilers. See, for example, theabove article by Hickman, Hickman U.S. Pat. No. 2,080,421, and "A NewType of Diffusion Pump Boiler for Ultrahigh Vacuum Use", H. Okamoto andY. Murakami, Vacuum, Volume 17, No. 2. The suggested solutions includethe use of a central purge sump within the boiler for segregatingcertain impurities which overflow therein during boiling; the use of aboiler heater designed to induce tremendous turbulence in the pump fluid(Stevenson Flash Boiler); and various means for circulating the pumpfluid in the boiler. The latter means include stirring or otherwiserotating the fluid mechanically, and positioning the applied heat so asto induce circulation (N-boiler of Murakami). Numerous diffusion pumpboiler modifications are shown and described at pages 974-976 of thefirst above-referenced Hickman article.

While then suggested solutions have reportedly increased molecularthroughput somewhat, they do not have the capability to purify the pumpfluid within the boiler thereby eliminating the causes of torpidity, asdiscussed below. The exception is the purge sump, which does remove alimited quantity of impurities from the surface of the evaporatingliquid. As long as torpid areas of the evaporative surface prevail,molecular throughput and attainable vacuum remain drastically limited.

Another limitation in diffusion pumps on ultimate attainable vacuum isimposed by a phenomenon known as "backstreaming". Backstreaming, alsoknown as "reverse fractionation", constitutes a back migration of somemolecules from the jets back into the vacuum chamber and is inherent ina diffusion pumping process. As pressure in the chamber being evacuateddecreases, the rate of backstreaming increases, and when it equals thethroughput of gas, no further decrease in chamber pressure occurs. Thephenomenon of backstreaming, and various suggested remedies therefor,are discussed in Hickman U.S. Pat. Nos. 3,034,700 and 2,080,421,Scatchard U.S. Pat. No. 2,905,374, Nelson U.S. Pat. No. 2,291,054,Bachler U.S. Pat. No. 3,317,122 and Hayashi U.S. Pat. No. 3,171,584. Forexample, in Hickman U.S. Pat. No. 3,034,700 and in Bachler U.S. Pat. No.3,317,122 it is suggested that backstreaming can be reduced by coolingthe diffusion pump barrel only behind the jet or adjacent the upperstages of a multi-stage jet assembly, with the lower portions of thediffusion pump barrel being maintained warm. This reportedly maintains along column of forwardly moving pump fluid vapor, giving the moleculesless chance to diffuse backstream. Another often employed way ofreducing backstreaming is the use of one or more cryogenically cooledbaffles between the vacuum chamber and the pump. The baffle primarilyattempts to condense and trap contaminant gases from the chamber and toprevent diffusion pump vapors from backstreaming into the chamber. Manyof the chamber gases originate from materials in the chamber which have"outgassed" under the influence of high vacuum. While cooled baffleshave been helpful in trapping chamber gases and reducing backstreaming,they have not been able to trap all passing gases, and once they arefilled with condensate, they lose their effectiveness. If the baffle iswarmed, the condensables drip into the boiler and cannot be removed.

Although molecules of the pump fluid itself exiting the diffusion pumpjets have a tendency to backstream to a slight degree, it is primarilymolecules of "light ends" which backstream through the diffusion pumpbarrel toward the lower pressure vacuum chamber, thus severely limitingthe degree of vacuum attainable. "Light ends" are those contaminantspresent with the pump fluid which are of lower molecular weight than thepump fluid itself, and may include broken away fractions of molecules ofthe pump fluid itself, which is, in the case of the pump fluid DC-705, apentamer. One system for partially removing light end contaminants fromthe pump fluid, thereby reducing backstreaming, is shown in Hickman U.S.Pat. No. 3,034,700, FIG. 1, and in the first above-referenced Hickmanarticle, FIG. 25 and page 976. This system consists of collectingcondensed distillate in annular alembics defined in the foreline of thediffusion pump. The distillates comprise light end substances which haveescaped the diffusion pump barrel and passed along with vacuum chambergases into the foreline.

In Hickman U.S. Pat. No. 2,080,421, wherein total pump fluidconstituents, including impurities, are identified by letters A throughZ from the lightest light ends through the heaviest ends, diffusion pumpapparatus is disclosed wherein certain impurities were isolated withininternal compartments. However, only contaminating components A, B and Zare disclosed as having been successfully isolated.

While the diffusion pump structures discussed and referenced above aidin the reduction of torpidity on the fluid's evaporative surface withinthe boiler and in the reduction of backstreaming by light end substancesinto the vacuum chamber, thereby increasing ultimate attainable vacuum,the suggested structures cannot produce a 100% continuously workingevaporative surface, nor reduce backstreaming and achieve high fluidseparation to the extent of the present invention described below.

SUMMARY OF THE INVENTION

The present invention provides a diffusion pump, including modificationsto existing large diffusion pumps, capable of continually cleansing theworking fluid during operation, thereby producing a continuous, 100%working evaporative surface, as well as drastically reducingbackstreaming, which heretofore has severely limited ultimate attainablevacuum. The diffusion pump apparatus of the invention is also capable ofachieving a high degree of separation among fluids of slightly differentvolatility, and thus has utility in the field of refining and other artsinvolving high purification and separation of fluids.

It has been found that the torpidity phenomena, which has been thesubject of study for some years as discussed above, is due almostentirely to the presence of "heavy end" contaminating substances presentin the pump fluid. The heavy ends comprise substances of molecularweight higher than that of the pump fluid, and include to a large extentpolymerized molecules of ligher substances. These heavy ends tend tocollect into nonevaporating "islands" on the evaporative surface,restraining the release of motive fluid molecules in these areas. Thus,only the working "holes" in the schizoid evaporative surface releasefluid molecules at an appreciable rate.

The diffusion pump apparatus of the present invention is capable ofalmost totally eliminating heavy end substances from the pump fluidduring operation of the pump, thereby removing the principal cause oftorpidity on the evaporative surface. This helps provide for a vigorous,100% working surface, resulting in a much greater molecular throughputand greatly improved pump efficiency. The elimination of heavy ends isaccomplished in part by the periodic skimming of the evaporative surfaceof the pump fluid in the boiler during operation. Skimming outlets arelocated around the periphery of the boiler and also toward the center ofthe boiler in large pumps having concentric annular boiler channels. Theskimming openings are provided at various levels on the boiler and areconnected, when skimming valves are opened, to a collection vesselexisting at lower internal pressure than that of the boiler.

The diffusion pump apparatus of the invention also includes means forwithdrawing condensed distillate from the forepressure line of the pump.Alembics are provided in the foreline for catching condensates flowingdown the internal walls thereof. Valved lines connect the alembics witha collection vessel of lower internal pressure than that of theforeline. Condensates within the foreline contain high concentrations of"light end" substances, and the removal of these light ends preventstheir return to the boiler for revaporization and possiblepolymerization into heavy end molecules. In this way, backstreaming isnearly eliminated, since most light ends are prevented from re-enteringthe flow of vapors through the diffusion pump jets. The presence oflight ends in the diffusion pump barrel is the primary cause ofbackstreaming, which greatly reduces ultimate attainable vacuum in avacuum chamber-diffusion pump system. As indicated above, the preventionof light ends from returning to the boiler also reduces torpidity byeliminating polymerized molecules therefrom.

Light end contaminants cannot be completely eliminated from the systemvia the foreline. Usually there is present a very small quantity oflight ends of molecular weight only slightly above that of the pumpfluid itself. For maximum pump performance, these substances must beremoved. They are difficult to isolate in the foreline, where pressureis relatively high and pressure differences are only slight, and wherethe concentration of such heavy light ends is generally not as great asit is in the pump barrel. The heavy light ends, not being as volatile asthe lighter light ends which may be isolated in the foreline, condenseon the diffusion pump barrel walls nearly as readily as does the motivefluid. Therefore, molecules of the heavy light ends are continuallypresent in the diffusion pump barrel and available for backstreamingtoward the vacuum chamber and for combining by polymerization to formheavy end contaminants which cause torpidity in the boiler. Beingsomewhat more volatile than the pump fluid itself, the uncombined heavylight end molecules tend to backstream more readily than those of thepump fluid, toward the lower pressure of the vacuum chamber. If afterseparation and removal of most light light ends from the foreline,pressure could be made sufficiently low, with a sharper gradient, andtemperature sufficiently high in the foreline, the heavy light endscould be drawn in greater concentration into the foreline beforecondensation, condensed therein and trapped within the alembics. Thesubstances would thus be present in high concentrations within thecondensate and could be drawn off from the alembics. However, to achievesuch separation would require an extremely strong roughing or backingpump connected to the foreline, and extremely high temperatures withinthe foreline. In most situations, such a large vacuum pump would berequired to achieve the separation as to be impracticable andeconomically unfeasible. Also, any ligher light ends present at the timeunder such conditions would necessarily be drawn away through theroughing pump and exhausted. These light light ends would thus be lostand unavailable for separation and analysis or salvage.

A solution to the problem of the trace quantities of heavy light endspresent in the diffusion pump barrel is provided according to thepresent invention. On the internal surface of the diffusion pump barrelwhere pressure is low and a strong pressure gradient exists, are aseries of alembic-like plates or gutters which are positioned so as totrap a major portion of the heavy light end substances which havecondensed on the barrel surface, before these condensates have a chanceto revaporize and backstream toward the vacuum chamber. Valved draw offlines lead from each of the gutters to a collection vessel of lowerpressure than the barrel pressure at the level of that particulargutter. Certain of the gutters are provided with draw off lines at morethan one level, so that motive fluid condensate, which may be present inthe bottom of certain gutters, can be returned to the boiler, whilecondensates containing heavy light end contaminants can be drawn offfrom upper levels of the gutters into a collection vessel.

In a vacuum system including a number of diffusion pumps, one pumphaving all of the above-discussed modifications can act as a "slave"pump to the remaining pumps having only boiler and forelinemodifications. Boiler fluid would be exchanged between the pumps so thatthe fluid of each pump would reach a high degree of purification.

As discussed above, the apparatus of the present invention also hasutility as a precision fluid separation device. Apparatus describedbelow for boiler surface skimming and for collecting and separatingcondensates in the foreline and on the barrel of a diffusion pump can beutilized in other separation arts apart from diffusion pumps, such asthose relating to the separation of petroleum fractions, uraniumisotopes, metals, and other organic and inorganic chemicals.

The diffusion pump apparatus of this invention is illustrated anddescribed herein in connection with an umbrella or mushroom typeinverted-nozzle pump of the type shown in U.S. Pat. Nos. 2,206,093,2,386,298, 2,436,849, 2,905,374, 3,251,537, 3,317,122 and 3,536,420. Amodel diffusion pump embodying some of the improvements of the inventionwas constructed and operated, the results of such operation beingdiscussed in National Aeronautics and Space Administration TechnicalMemorandum X-68272, published Nov. 12, 1973 and presented at the SeventhSpace Simulation Conference, Los Angeles, California, Nov. 12-14, 1973.Remaining improvements according to the invention are discussed in NASATechnical Memorandum X-2932, to be published in or prior to January,1975.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view showing a vacuum chamber andconnected model diffusion pump including apparatus according to thepresent invention;

FIG. 2 is an enlarged view of the model diffusion pump;

FIG. 3 is a sectional elevational view of a typical prior art largemushroom-type diffusion pump;

FIG. 4 is a sectioned perspective view of the boiler of a typical priorart large diffusion pump;

FIG. 5 is a sectional elevational view showing modifications to a priorart diffusion pump according to the present invention;

FIG. 6 is a sectional plan view taken along the line 6--6 of FIG. 5; and

FIG. 7 is an enlarged sectional elevational view taken along the lines7--7 of FIG. 6 with parts removed fro clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A. LABORATORY MODEL DIFFUSIONPUMP

FIG. 1 of the drawings shows a vacuum chamber 10 connected to the lowpressure side of a model self-cleansing diffusion pump assemblyaccording to the invention, generally indicated by the reference number11. The assembly 11 includes a diffusion pump barrel 12, a chimney andnozzle assembly 13, a boiler 14, a forepressure line 16 includingalembics 17 and valved draw off lines 18, a line 19 leading through aforeline cold trap 21 to a mechanical backing pump 22, vessels 23 and 24for collecting contaminants removed from the boiler 14 and the foreline16 respectively, a foreline pressure gauge 26, a blower 27 for coolingthe diffusion pump barrel 12, and a boiler contaminant adding device 28.Included on the vacuum chamber are a pressure gauge 29, a chambercontaminant adding device 31 including a valve 32, an auxiliary bleedvalve 33, and a chamber baffle 34 which may be chilled to provide arefrigerated condensing trap for gaseous substances travelling towardthe diffusion pump barrel 12. An additional baffle 37 and valve 38 maybe provided between the valve 36 and the diffusion pump barrel 12. Thecontaminant collection vessels 23 and 24 are connected by valved lines39 and 41 to a source of lower pressure than that existing within thediffusion pump boiler 14 and foreline 16 so that the vessels 23 and 24will draw liquids from the boiler 14 and the alembics 17, respectively,when the appropriate valves are opened. The vacuum source may be themachanical backing pump 22 itself or some other suitable source, sincethe pressure existing within the boiler 14 and foreline 16 duringoperation of the pump assembly 11 are not extremely low, as is thepressure within the upper end of the diffusion pump barrel 12.Contaminant drain valves 35 and 36 are provided on the vessels 23 and 24for removing contaminants when the vessels 23 and 24 are appropriatelyisolated from the source of vacuum and from the boiler and foreline.Atmospheric bleed valves 40 and 45 are also provided on the vessels 23and 24 for the same purpose.

A portion of the diffusion pump assembly 11 is shown in greater detailin FIG. 2. The boiler 14 contains motive fluid 42 up to the level of askimming drain line 43 which is open to the interior of the boiler. Theline 43 can be opened by a valve 44 to a line 46 leading to thecollection vessel 23. On the opposite side of the boiler 14 is thecontaminant adding device 28 which includes an open-ended graduated tube47 and a valve 48 for admitting liquids from the tube 47 into theinterior of the boiler 14. Such contaminating liquids may be added todetermine their effect on pump performance and the ability of thepurification apparatus of the model pump to separate the liquids out ofthe pump fluid, as discussed below. The device 28 may also be used toreplenish pump fluid in the boiler 14 as heavy and light endconstituents are skimmed and drawn off. The fluid level shown in FIG. 2should be maintained through all skimming operations, since skimming isprovided only at one level in the model pump. The chimney and nozzleassembly 13 includes nozzles or jets 49a, 49b and 49c which are angleddownwardly to create, when pump fluid passes therethrough, a lowpressure on the high vacuum side 51 of the diffusion pump. At the bottomof the diffusion pump barrel 12, several openings 52 are provided aroundthe periphery of the chimney and nozzle assembly 13 so that any fluidcondensing on the lower interior of the chimney and nozzle assembly 13returns to the boiler 14 by passing into the foreline entrance andthence down through a conduit 53, through the line 43 and into theboiler 14. Similarly, condensate from the surface of the barrel 12returns to the boiler via the lines 53 and 43.

The foreline 16 of the diffusion pump includes a pair of peripheralalembics 17a and 17b, and lines 18a, 18b and 18c are provided to conductcondensed fluid away from the foreline 16. Each of the lines 18 may beopened by a valve to a light end collection vessel 24 of lower pressurethan that of the foreline 16, as shown in FIG. 1. The lower line 53,which remains open to the interior of the boiler 14, returns to theboiler those condensates which have not been removed through the lines18. Electric heating tape 54 is wrapped around the foreline 16 from justabove the line 53 to just below the backing pump line 19, so thattemperature within the foreline 16 can be maintained at a predetermined,nearly consistent level along the length of the foreline. This allowscondensable fluids entering the foreline in the gaseous state to travelfarther up the foreline 16 before condensing on the walls thereof, andwith temperature nearly constant along the foreline's length, light endsubstances of differing volatilities are allowed to condense atdifferent levels under the influence of subtle variation in pressurealong the length of the foreline 16. The highest pressure in theforeline is found adjacent the backing pump line 19, and the lowestpressure is found closest to the diffusion pump barrel 12.

As FIG. 2 indicates, a valved bottom drain line 56 is provided in theboiler 14 for draining the pump fluid 42 out of the boiler when thediffusion pump is not operating. The glass walls of the boiler 14include thermocouple inserts 57 and 58 for monitoring the temperature atvarious positions within the boiler 14. The thermocouple inserts 58extend from the back of the boiler. The foreline 16 also includes athermocouple insert 59 for monitoring temperature at the position shown.Foreline pressure is monitored via the pressure gauge 26 shown in FIG.1, while chamber pressure is monitored by the gauge 29, also shown inFIG. 1.

In a particular run of the model self-cleansing diffusion pump assembly11, five year old DC-705 silicone pump fluid (molecular weight 546)taken from the cyclotron diffusion pump at Michigan State University wasused in the boiler 14 of the model pump. The oil had been discolored bylong use to a dark brown color. In an initial standardizing testconducted with a standard G-4 single stage glass diffusion pump, thefive year old oil had attained a maximum vacuum, or minimum pressure, of2.5×10⁻⁵ torr (mm. of mercury) in four hours of pump operating time. NewDC-705 oil had achieved a minimum pressure of 1.0×10⁻⁶ torr in three andone-half hours operation in the G-4 test pump. The boiler size andevaporative surfaces area of a G-4 single stage pump are approximatelythe same as those of the model pump. Each pump has a boiler capacity ofabout 55 milliliters.

After one day of operation in the model diffusion pump shown in FIGS. 1and 2, the five year old DC-705 oil produced a maximum test chambervacuum of 5×10⁻⁵ torr. The reason for the difference in ultimate vacuumbetween the G-4 test pump and the model pump illustrated in the figures,using the same five year old pump oil, is primarily that the model pumpincluding a large number of joints between the diffusion pump barrel 12and the vacuum chamber 10 (some of which are seen in FIG. 1). Thesejoints were not perfect but allowed a small amount of gas leakage intothe system, consistently raising minimum attainable pressures in thesystem in all tests with the pump. Outgassing of gasket materials aroundjoints also contributed to the gas load in the system.

During the first day of operation of the five year old oil in the modeldiffusion pump, chamber pressure often varied from about 10⁻⁴ torr to5×10⁻⁵ torr. A torpid evaporative surface appeared approximately 80% ofthe time. For brief periods the evaporative surface would become 100%working. Between torpid and working periods, a surface previouslydescribed as schizoid would appear briefly, with numerous small working"holes" present in the otherwise torpid surface. Improved chamberpressure always accompanied the 100% working periods, often loweringchamber pressure by 0.5 decade (five times). The 100% workingevaporative surface appeared about 20% of the time during the first dayof operation.

When removal of light end and heavy end contaminants from the pump fluidcommenced, marked increases in pump performance immediately becameapparent. Within eight hours from the first separation of suchimpurities, chamber pressure was lowered by more than one decade to alevel of 1×10⁻⁶ to 5×10⁻⁶ torr. During approximately the second day ofpump operation, the following quantities of fluid containing theindicated contaminants were removed, in the indicated order:

10 ml light ends

3 ml light ends

8 ml heavy ends

2 ml light ends

2 ml light ends

Following the removal of these contaminants, a chamber pressure varyingbetween 8×10⁻⁷ and 1×10⁻⁶ torr was attained. The corresponding boilerevaporative surface behavior associated with this pump efficiency was100% working 90% of the time.

After seven days of system operation, chamber vacuum reached improvedlevels of between 5×10⁻⁷ and 1×10⁻⁶ torr. The 100% working evaporativesurface now appeared approximately 99% of the time.

During the eighth day, a further 2 ml of pump fluid containing lightends was removed from the foreline. Following this final purification, acontinuous 100% working evaporative surface appeared. Chamber pressureremained between 10⁻⁷ and 10⁻⁶ torr.

During this run of the model diffusion pump, when chamber pressure wasvarying from about 5×10⁻⁷ and 1×10⁻⁶ torr, a test to determine theeffectiveness of the chamber baffle 34 (see FIG. 1) was conducted. Thechamber baffle 34 had been maintained at temperatures varying between-40° F. and -20° F. When the baffle was warmed to a temperature of +40°F., chamber pressure rose to about 5×10⁻³ torr, due to vaporization ofwarmed condensates which had been detained by the chamber baffle.However, the system later recovered from this pressure rise, and in factwhen chamber baffle temperature exceeded 60° F., vacuums of around 10⁻⁶torr were again attained and maintained. These results confirm that whendiffusion pump oil is maintained in an extremely high purity condition,high vacuum can be attained merely with the use of a water cooledchamber baffle. Water cooled baffles could save enormous expense atspace performance testing facilities, for example, where each of a largenumber of diffusion pumps usually employs a chamber baffle cooled withcostly cryogens such as liquid nitrogen or liquid helium.

During the above test of the self-cleansing model pump, forelinepressure gradually and steadily decreased as chamber pressure decreasedand as the continuous 100% working evaporative surface condition wasapproached, from about 4×10⁻³ torr to about 2×10⁻³ torr. During thisperiod, foreline temperature, controlled by the electric heating tape 54shown in FIG. 2, remained at about 260° F. and did not varysignificantly. The gradual lowering of pressure in the foreline thussteadily changed the conditions of equilibrium within the foreline, suchthat a substance of a particular vapor pressure would condense at acontinually rising level in the foreline as pressure decreased. Thus aparticular substance which may have condensed near the entry to theforeline after only several hours or a day of pump operation (prior topurification of the pump fluid) may later condense just above the loweralembic 17b, being trapped therein. Still later, the same substance mayreach an even higher position of condensation within the foreline 16,thereby flowing into the upper alembic 17a for collection.

Because of this steady change of conditions, or equilibrium parameters,within the foreline, the lightest, most volatile light ends are thefirst light end contaminants to be collected in the forearm alembics 17aand 17b. The alembics are drained of these lightest light end substances(the drain line 18c may also be opened), and pressure in the forelineprogresses downwardly. The next cut of light ends from the forelineoccurs when pressure is somewhat lower, so that contaminants containedin the fluid removed from the alembics now comprise somewhat heavier,less volatile light light ends. If any of the lightest, most volatilelight ends are still present within the system, they would be likely tobe passed completely out of the foreline, through the line 19 toward themechanical backing pump, under the new conditions. Thus, the initiallight end cut should be sufficient to remove nearly all of these mostvolatile substances, particularly, if separation and salvage of thesubstances is desired. As the operation of the system progresses, lightend contaminants continue to be removed. Ultimately, the heaviest, leastvolatile light end substances can be largely removed from the foreline.As previously discussed, under any set of equilibrium conditionsexisting in the foreline at a given time, there is a slight variation inpressure along the length of the foreline 16. The pressure varies fromlowest at the upstream end of the foreline to the highest at thedownstream end adjacent the backing pump line 19. Light end substancesof varying volatilities can thus be drawn from the lines 18a, 18b and18c during operation of the pump.

After the initial drawing off of light ends and substantialstabilization of foreline pressure, further light end removal may beaccomplished by draining only the upper alembic 17a through its draw offline 18a. This results from the fact that once foreline pressure hasreached a minimum, primarily the heavier light ends are being trapped bythe alembics 17a and 17b. Most of the light end condensates within thelower alembics 17b will revaporize from time to time, travelling fartherup the forearm to be eventually recondensed higher along the walls ofthe foreline 16, adjacent the backing pump line 19 where temperature issomewhat lower. This, most of such condensates eventually are trapped bythe upper alembic 17a.

The model self-cleansing diffusion pump system shown in FIGS. 1 and 2was also tested using new DC-705 oil. After three days of operation, thevacuum chamber pressure gauge 29 indicated chamber pressure to be aconsistent 5×10⁻⁵ torr. A 100% working evaporative surface existed about10% of the time. When 6 ml of fluid containing light end impurities wereremoved from the foreline, 2 ml from each of the drain lines 18a, 18band 18c, chamber pressure fell by 0.6 decade. The evaporative surfacebecame 100% working 25% of the time. On the fifth day, the removal ofapproximately 7 ml of boiler fluid containing heavy end fractions, byskimming the evaporative surface, increased the 100% working surfacecondition to 50% of the time, with a slight reduction in chamberpressure. Following an additional removal of 3 ml of light endscontaining condensate from the top alembic, on the sixth day, chamberpressure decreased, reaching an ultimate level of from 7×10.sup. -7 to2×10⁻⁶ torr. Eventually a continuous 100% working evaporative surfacewas achieved, following a further removal of 4 ml fluid from the upperalembic in the foreline.

The foregoing test provided convincing evidence that even new unusedDC-705 silicone oil contained some light end and some heavy endimpurities, and that the model pump 11 is capable of a very high degreeof separation and purification. Minimum pressure attainable in the testchamber was reduced from about 5×10⁻⁵ torr to about 1×10⁻⁶ torr, orabout 1.5 decades, by the removal of light end and heavy end substancesduring operation of the pump.

Other tests were conducted using the model self-cleansing diffusion pumpassembly illustrated in FIGS. 1 and 2. In one test, new DC-705 oil whichhad been further purified in the model pump as described above, wasdeliberately contaminated to determine the effect of certaincontaminants on pump operation, and the ability of the model pump toeliminate them. This test was actually a continuation of the above runof the pump using new DC-705 oil, beginning with the 13th day ofoperation. The contaminants added were three common phthalateplasticizers which have been found to outgas from many of the articlespresent in a space simulation chamber. The three phthalate contaminantsadded were: di-isooctyl phthalate (DIOP-390 molecular weight);di-isodecyl phthalate (DIDP-446 M.W.); and di-octylphthalate/di-2-ethylhexyl phthalate (DOP/DEHP-390 M.W.). 2 ml of eachcontaminant were added directly into the pump fluid through the boilercontaminant adding device 28. The effect was an immediate, sharp rise inchamber pressure from about 10⁻⁶ torr to about 2×10⁻³ torr, with chamberpressure stabilizing at about 7×10⁻⁴ torr. With the removal of 4 mlfluid from the top alembic of the foreline, chamber pressure dropped to2.5×10⁻⁵ within one hour. The insertion into the boiler of an additional2 ml of phthalate contaminant mixture (containing the above threecomponents), the previous pressure fluctuation was duplicated. 10 ml ofnew DC-705 was added through the device 28 to prevent depletion of theboiler fluid supply during the pending cleansing period. To purify thefluid, 2 ml of fluid containing light end fractions were removed fromthe upper alembic 17a of the forearm 16. This operation was repeatedagain within 30 minutes. Within one hour, chamber pressure was reduced 3decades, from 3×10⁻³ torr to 2.5×10⁻⁶ torr. 15 hours later, a chamberpressure of 8.5×10⁻⁷ torr was achieved.

The operation of the model diffusion pump assembly described aboveestablished that a high level of motive fluid purity can be attained andmaintained by utilization of the contaminant removal apparatus shown anddescribed. The model pump further demonstrated that once heavy endcontaminants have been initially skimmed from the evaporative surface inthe pump boiler, heavy and light end contaminants both can be controlledto a large extent merely by periodic withdrawal of condensates from theforeline. This is due to the fact that after an initial skimming mostrecurring heavy ends in the boiler comprise polymerized molecules madeup of light end molecules or light end molecules in combination withmolecules of the basic motive fluid.

As discussed above, the model diffusion pump was subject to a number oflimitations. Among these limitations were the large number of jointsemployed in the vicinity of the diffusion pump barrel and the foreline,and the size of the model diffusion pump. Nonetheless, the separationand purification capabilities of the model pump demonstrated thatgreatly improved pump performance can be attained employing the twotypes of modifications shown. An even higher degree of purification andcontaminant separation can be attained with a third diffusion pumpmodification described below in connection with large existing pumps.The third modification, which could not be incorporated in the glassmodel pump, relates to apparatus within the diffusion pump barrel itselffor separating out the extremely small quantities of light endsubstances which are only slightly below the molecular weight andslightly above the vapor pressure of the motive fluid itself. Theseparation of this type impurity cannot be completely accomplishedwithin the diffusion pump foreline.

B. MODIFICATION OF LARGE DIFFUSION PUMPS

A typical large diffusion pump 65 of the mushroom or inverted-nozzletype is shown in FIG. 3. The pump 65 includes a concentric channelboiler 66, disposed at the bottom of a chimney and nozzle assembly 67.The boiler 65 includes electrical resistance heating elements 68disposed between concentric boiler channels for providing a largeheating area, and a metal dome 69 receives conductive heat from theelectrical heating elements 68 for assuring the vaporization of pumpfluid striking it. The boiler includes a valve drain line 71.Surrounding the chimney and nozzle assembly 67 is a barrel 72 aboutwhich a cooling jacket 73 containing cooling lines 74 is usuallywrapped. The pump includes a foreline elbow pipe 75 leading through aforeline (not shown) to a mechanical backing pump (not shown).

Referring to FIG. 4, the boiler 66 of the typical prior art diffusionpump 65 of FIG. 3 is shown in greater detail. The boiler 66 includesthree separate concentric annular boiler channels 78a, 78b and 78c. Theelectrical resistance heating elements 68 are enclosed within conductivemetal heating rings 79a and 79b, and 78b and 78c. Communication isprovided among all the boiler channels 78 by a break in the heatingrings 79a and 79b. As seen at the left in FIG. 4, the rings 79a and 79bterminate in the vicinity of the drain line 71 and thus are C-shaped. Anannular flange 81 extends into the boiler from the chimney and nozzleassembly 67 above to prevent the travel of vaporized pump fluid directlyinto the diffusion pump barrel 72 rather than through the chimney 57(see FIG. 3). The flange 81 does not reach the boiler bottom. The boiler66 is designed to maximize contact between the motive fluid and heatedsurfaces. Some boilers include additional heating fins extending throughseveral of the boiler channels 78 in zigzag fashion for additionallyincreased surface contact.

FIG. 5 shows a diffusion pump assembly 85 similar to the prior artdiffusion pump 65 shown in FIGS. 3 and 4 but including self-purificationand separation modifications according to the invention. The modifiedpump 85 includes a boiler 86 with electrical resistance heating elementrings 87, a dome 88, a filling and drain line 89, a chimney and nozzleassembly 90, a barrel 91 encircled by cooling jackets 92 and a foreline93 connected through an opening 94 to a mechanical backing pump (notshown). Heating means is provided about the foreline 93, and maycomprise electric heating tape 95 wrapped around the foreline andcovered by an insolating jacket 95a.

As indicated in FIG. 5, the boiler 86 of the diffusion pump 85 ismodified to provide evaporative surface skimming means from inner andouter positions. On the inner side of the boiler 86 within the dome 88are skimming tubes 96a, 96b, 97a, 98a and 99a, communicating throughopenings in the base of the dome 88 with the interior of the boiler atpreferably four different levels. Opposite skimming tubes 97b, 98b and99b at the levels of 97a, 98a and 99a, respectively, are seen in FIG. 6.All of the skimming tubes are connected through valves 100 with a heavyend collection vessel 101. The vessel 101 is connected through a valveline 102 with a source of lower pressure than that within the boiler 86.A valved drain line 103 and a valved atmospheric bleed line 104 are alsoprovided for periodically draining the liquid from the collection vessel101. Several outer skimming lines 106a, 106b, 107a and 108a are alsoseen in FIG. 5. These outer skimming lines are connected to a separateheavy end collection vessel 109. The vessel 109, like the vessel 101,has valved lines 111, 112 and 113 connected to a vacuum source, a drainsump and the atmosphere, respectively. The reason for separatecollection vessels 101 and 109 for inner and outer boiler-skimmedresidues is that heavier heavy ends, which largely comprise polymerizedlight ends, tend to be higher in concentration toward the boiler'speriphery, where temperature is somewhat lower due to the presence ofreturned condensates from the barrel wall. Lighter heavy ends remaintoward the boiler center. Thus, the skimming apparatus herein describedprovides for separation between the two ranges of heavy ends, ifdesired.

FIGS. 6 and 7 indicate the various positions of the boiler skimminglines. An additional valved outer line 114a, not seen in FIG. 5,connects with the back side of the boiler 86, with a frontal line 114bat the same level. Lines 107b and 108b extend from the front of theboiler 88 at the level of lines 107a and 108a, respectively. The innerand outer skimming lines provide for boiler surface skimming atpreferably four different levels, as indicated in FIG. 7. Inner lines96a and 96b and outer lines 106a and 106b are preferably positioned atthe level of full boiler capacity, which is the four gallon level inmany typical diffusion pumps. An opening 105 is provided in the chimneyflange 110 (similar to flange 81 of FIG. 4 pump) of the pump 85 toprovide for surface communication throughout the boiler 86, as seen inFIG. 6. At preferably about 5 millimeters below the boiler capacitylevel are the inner skimming tubes 97a and 97b and the outer skimmingtubes 114a and 114b. The tubes 98a, 98b, 107a and 107b are preferablyabout 10 millimeters below the capacity level, while the tubes 99a, 99b,108a and 108b are at about 15 millimeters below the capacity boilerlevel. The multiple skimming levels provide for surface skimming whenthe pump fluid is at various levels below full, as well as at the fulllevel. This allows for the loss of portions of the pump fluid aftervarious stages of contaminant removal. Means (not shown) can be providedfor automatically opening the proper skimming lines in correlation withthe boiler fluid level, which can be ascertained by a sensor. Means (notshown) can also be provided for determining when skimming is required,by sensing evaporative surface behavior and/or vacuum chamber pressure.

Referring again to FIG. 5, the foreline 93 of the modified diffusionpump 85 is shown with purification and separation modifications. Theinterior surface of the foreline 93 includes a series of alembic-likegutters or troughs 116, 117, 118, 119 and 120. In addition, an elbow 122of the foreline includes traps 123, 124 and 125 for catching condensatesas they flow down along the elbow wall toward the boiler. The varieslevels for condensate removal, from the top to the bottom of theforeline, provide for trapping of condensates of varying volatility, asdiscussed in connection with the model pump, and separate collectionthereof. Two valved lines, referenced by 116a and 116b, 117a and 117b,etc., extend from an upper level and from the bottom of each gutter andtrap, respectively. The reason for the bi-level fluid outlets is thatthe bottom of each gutter or trap, particularly in the traps and lowergutters, will usually contain nearly pure motive fluid. On the otherhand, the upper levels of the gutters and traps will contain fluidhaving high concentrations of condensed light end substances. Thus, thelines 116b, 117b, 118b, etc., which lead into a collection vessel 127,are used primarily for recovering the basic motive fluid from thegutters and traps. The vessel 127 includes a valved drain line 128 andvalved lines 129 and 130 connected to a source of lower pressure thanthat of the foreline and to the atmosphere, respectively. The valvedlines 116a, 117a, 118a, etc., from the upper levels of the gutters andtraps may be connected to separate collection vessels (not shown) ifseparation of fluids containing light end contaminants of varyingvolatilities is desired. Each collection vessel would be provided with adrain line, a line leading to the source of lower pressure, and a bleedline to the atmosphere, as with the vessel 127. If separation is notdesired, the upper level gutter and trap line may lead to a commoncollection vessel.

For maximum fluid separation in the foreline 93, with somewhat loweredthroughput and consequently reduced vacuum production, each of thegutters 116 through 120 can be provided with a condensing baffle above(not shown for clarity). In addition, packing (not shown) can beprovided in each gutter to increase effective condensing area. Suchpacking might comprise, for example, stainless steel or another inertmaterial in wire mesh or finely spun form.

A heating jacket 132 is positioned about the entire foreline 93 andfunctions in the same manner as the heating tape described above inconnection with the model diffusion pump assembly 11. In addition tothis function of bringing light end substances to high enough levelswithin the foreline for condensation and trapping in appropriategutters, the heating jacket 132 also aids in the separation of lightends from the motive fluid within the gutters and traps, and theirwithdrawal through the lines 116a, 117a, 118a, etc. At each gutter, theforeline wall is warmer than the gutter itself and other points withinthe interior of the foreline. This causes fluids contacting the forelinewall to convect upwardly along the foreline wall, thus creating acirculation pattern within the gutter. The fluid circulation pattern isup along the wall, inward along the surface of the fluid, and thendownward and outward along the bottom of the gutter toward the wall.This circulation pattern aids in bringing the lighter fluids toward thesurface of the collected condensate. Since the light end fluid withineach gutter is at conditions very close to its liquid-vapor equilibriumconditions, the light end condensate tends to move toward the liquidsurface for incipient vaporization and actual vaporization to someextent. Polymerization of these light end fractions at gutter surfacesmay also play an important part as to the ease in which a specificfraction is drawn from the surface into a vessel of lower pressure.

As discussed in connection with the model self-cleansing diffusion pumpassembly 11, at the initial stages of pump operation the lightest lightend contaminants are trapped within the lower traps 123, 124 and 125.This occurs because pressure within the foreline is relatively high, nothaving yet been effected by increased pump performance and progressivelyhigher vacuum within the vacuum chamber connected to the low pressureside of the diffusion pump barrel 91. These light light ends should beinitially removed as completely as possible via the lines 125a, 124a and123a.

As in the operation of the model diffusion pump, progressively loweredpressures within the foreline 93 cause progressive changes in theaverage position of condensation for a given light end cantaminant.Thus, the light end-containing liquids drawn through the lines 125a,124a and 123a initially will be higher in volatility than those laterdrawn therethrough, and if each fraction is to be separately collected,the collection vessels for each of the drain lines 125a, 124a and 123awill have to be drained at intervals. As in the model self-cleansingpump, once pressure within the foreline 93 has stabilized at minimumvalues, withdrawal of nearly all light end contaminants can beaccomplished through one or several of the uppermost gutters 116a, 117aand 118a. This is because as maximum foreline vacuum is reached, onlythe heavier light ends will be condensing in the gutters. The lighterlight ends will have been previously removed, or if any remain, willprimarily be passed out of the foreline through the opening 94 towardthe mechanical backing pump. At this point, many of the light ends inthe lower gutters 120, 119 and 118 tend to revaporize from time to timeand travel farther up the foreline, eventually recondensing in a higherlevel gutter and enriching the concentration of light end contaminant inthat gutter. Thus most light ends remaining at this point can be removedthrough the upper gutter or gutters.

In the initial stages of operation of the diffusion pump 85 includingthe above-described foreline modification, the preferred procedure forlight end contaminant removal is the sequential opening of the "a"skimming lines, beginning with the trap line 125a and ending with theuppermost gutter line 116a. the withdrawn condensates of each gutter ortrap can then be analyzed to determine the distribution pattern of lightend contaminants for a given pump fluid and a given set of chamberconditions after a given period of time of pump operation. Thisoperation can then be repeated after various periods of time, and theresults can be used to automate the removal of foreline light endcontaminants. According to the results, the valves of the lines 125a,124a, etc., through 116a can be programmed to be opened at predeterminedtimes and for predetermined time periods for maximum light endcontaminant removal and minimum withdrawal of the basic motive fluid.After conditions have been altered by increased pump performance andforeline pressure has dropped, as discussed above, fluid removal fromthe upper gutter line or lines can be timed to occur periodicallyaccording to past performance and analysis of withdrawn forelinecondensates, or such removal can be made responsive to changes in vacuumchamber pressure. For example, if chamber pressure rises and issustained at the higher level, fluid could be removed from the uppermostgutter line 116a for several seconds, since such increased pressurewould indicate the presence of volatile contaminants. If a sensordetermines that chamber pressure is fluctuating, indicating the probableoccurrence of alternate periods of working and torpid evaporativesurface, the presence of heavy end contaminants within the boiler wouldbe indicated. As discussed above, after an early initial heavy endboiler skimming operation, recurring heavy ends in the boiler are likelyto be primarily made up of polymerized light ends. Therefore, when suchchamber pressure fluctuations are indicated, the foreline skimmingvalves can again be programmed to remove fluid from the uppermost gutter116a. If the pressure fluctuations then persist, a second boilerskimming operation from the appropriate level could be mandated by theautomatic system.

During periods when light end fluids are not being removed from theforeline, the series of gutters and traps creates a waterfall effectwithin the foreline. Each gutter fills with condensate and overflowsinto the next gutter or trap, and so on, eventually flowing through theelbow 122 of the foreline and returning to the boiler. In fact, thewaterfall effect prevails continually, involving all gutters exceptthose being drained at any given time.

It should be pointed out that fluids withdrawn from the traps andgutters of the foreline 93 will never be 100% pure light endcontaminants. There will always be a significant portion of basic motivefluid within such withdrawn fluid. The apparatus described effects theenrichment of a particular light end contaminant in a particular gutterof the forearm, enabling the eventual removal of nearly all of thatcontaminant. As indicated above, the proper sequencing and timing ofgutter fluid removal can result in a minimum removal of motive fluidusing the multiple-gutter structure described. If the motive fluidwithin the withdrawn condensates is to be salvaged or if the light endcontaminant from a particular gutter is desired in a more concentratedform, the withdrawn condensates can be separated by another means, suchas a centrifugal molecular still as described in the above-referencedNASA Technical Memorandum X-68272.

For an additionally high degree of separation and purification notattainable with the above-discussed foreline and boiler modificationsalone, diffusion pump barrel modifications shown in FIG. 5 may also bemade. As indicated, the diffusion pump barrel 91 may include around itsinner periphery condensate collection gutters 140, 150, 160 and 170.Between the gutters on the outside of the diffusion pump barrel 91 arethe cooling jackets 92 which cool the barrel 91 for condensing themotive fluid and various contaminating fluids thereon. The coolingjackets 92 are interrupted at the level of each gutter for the provisionof a heating band 134 which extends around each gutter and is brokenonly at the location of draw off lines described below. The bands 134may comprise electric heating tape 95 employed to heat the foreline 93,around the barrel wall at each gutter.

The modified diffusion pump barrel 91 acts as a highpurification-separation still, as does the modified foreline 93.However, the modified barrel 91 is capable of attaining a much higherdegree of purification and separation than is the foreline 93, after theappropriate separation has been made by fluid withdrawal from theforeline gutters. After such separation, virtually the only remaininglight end contaminants should be those very close in volatility to thebasic motive fluid itself. As discussed above, foreline modificationscan only remove a portion of these heavy light end contaminants, leavingthe rest to backstream within the pump barrel and reduce ultimateattainable chamber vacuum to some extent. Since, as discussed above inconnection with the analysis of used five year old DC-705 oil taken fromthe cyclotron diffusion pump at Michigan State University, the quantityof heavy light ends close in volatility and molecular weight to themotive fluid is quite small, the diffusion pump barrel modificationsdescribed herein are designed to separate out extremely minutequantities of contaminants within the diffusion pump barrel, possibly inthe parts per billion range. Foreline purification is conducted first inorder that substantially only heavy light end contaminants remain in thebarrel along with the motive fluid.

The ultra-high purification capability of the modified diffusion pumpbarrel 91 depends in part on the existence therein of large variationsin pressure at different levels. In the foreline 93 only microns ortenths of microns pressure difference exists throughout the forelinelength, whereas decades of pressure difference exists in the barrel 91.For example, in the diffusion pump 85 of FIG. 5, pressure above the topgutter 140 after foreline purification may be around 10⁻⁸ torr, butaround 10⁻⁷ torr near the gutter 150, in the 10⁻⁶ or 10⁻⁵ torr rangeadjacent the next lower gutter 160, and around 10⁻⁴ torr near the lowestgutter 170. The high degree of separation discussed can be achieved byutilizing this strong pressure gradient to condense heavy light endsubstances at varying levels depending upon volatility, and to separatesuch contaminants from the motive fluid itself. The operation is verysimilar to that of the modified foreline 93.

As in the foreline 93 as discussed above, the barrel gutters 140, 150,160 and 170 can be provided with packing (not shown) for increasedcondensing area. Such packing can increase separation capabilities inthe pump barrel 91.

A valved low pressure line 135 extends from the diffusion pump barrel 91at a point above the highest gutter 140, where pressure is lower thanthat existing anywhere below in the diffusion pump barrel. This lowpressure line 135 extends downward to serve a fluid collection systemfor each gutter in the manner described below. In addition, a valvedline 136 extends from the lower end of the line 135, leading to aholding vacuum pump (not shown) which may be the same mechanical backingpump that is connected to the foreline 93. This pump providespreliminary pressure reduction for the collection system and in fact canserve one or several of the lower gutters by itself, since these guttersare at higher pressure ranges. A valve 137 in the line 135, which may bepositioned as shown or higher or lower, depending upon pressuresexisting in the barrel 91, is provided toward this end.

At an upper position in the uppermost gutter 140, a valved fluid drawoff line 141 is positioned to deliver fluid into a collection vessel142. The line 141, when opened, withdraws condensates only from adjacentthe upper surface of the fluid in the gutter 140, for reasons similar tothose discussed in connection with the foreline modifications above.Since the collection vessel 142 must be at lower pressure than thepressure within the barrel 91 adjacent the gutter 140 in order towithdraw fluid from the gutter 140, it is connected by a valved line 143to the low pressure line 135. The vessel 142, like all low pressurecollection vessels previously described, is also provided with a valveddrain line 144 and an atmospheric bleed line 145, which must be openedin order to drain the vessel 142. Extending from the bottom level of thegutter 140 is a valved gutter drain line 146 which connects into a lowergutter draw off line 151.

To withdraw surface condensate from the gutter 140, the line 143 isfirst opened to lower the pressure in the vessel 142. Then the line 141is opened to draw fluid into the vessel 142. To drain the vessel 142,the lines 141 and 143 are closed, and the lines 144 and 145 are opened.

The fluid collection structure for the next lower barrel gutter 150 issimilar to that above, with the reference numbers 151-156 describingsimilar elements. Fluid drained from the lower portion of the gutter 140through the line 146 may be drawn into the collection vessel 152 alongwith fluid from the top of the gutter 150 or drained into the gutter 150to enrich the concentration therein of the particular fraction presentin the gutter 140 bottom. This can be accomplished by sizing of the line146 such that liquid is able to drip down therethrough, even thoughthere is a slight pressure differential between the gutters 140 and 150.Alternatively, after the surface condensate in the gutter 140 has beensubstantially withdrawn, the gutter 140 may be allowed to overflow intothe gutter 150, thereby enriching the gutter 150 in the condensates ofthe gutter bottom 140.

The remaining diffusion pump barrel gutters 160 and 170 have fluidcollection apparatus similar to that of the gutters 140 and 150 andnumbered accordingly. The valved gutter drain line 176 leading from thelowermost gutter 170 is connected into a collection vessel 182 having avalved line 183 leading to the low pressure line 135 and having drainand bleed lines 184 and 185 similar to those above.

Toward the bottom of the diffusion pump barrel 91, a valved auxiliaryboiler fluid add line 187 is provided, the purpose of which is describedbelow.

In the initial operation of the diffusion pump assembly 85 includingbarrel modifications, all valves shown on the right side of the barrel91 in FIG. 5, from the line 135 through the line 185, are normallyclosed. The valves all remain closed during foreline condensate removaloperations. Prior to any condensate removal from the pump barrel 91, itis estimated that the motive fluid is between 98% and 99.8% pure.Maximum chamber vacuum attainable by boiler skimming and forelinepurification has been attained, and may range, depending upon the motivefluid employed, from about 10⁻⁵ to 10⁻⁹ torr. Pump fluid expelled fromthe chimney and nozzle assembly 90 would of course continually befilling up the lower three gutters 150, 160 and 170. The upper gutter140 is thus the primary location for removing backstreaming heavy lightend contaminants. Most of such heavy light ends reach the barrel wallabove the upper gutter 140 by backstreaming upward from lower pointsbeneath the jets. To a much lesser extent, some of the heavy light endsubstances reach the gutter 140 as a result of outgassing in the vacuumchamber above.

Heavy light end molecules of course also condense lower in the barrel 91and are collected in the gutters 150, 160 and 170. Some of thesecondensates can be recovered by surface removal through the lines 151,161 and 171, such removal being aided by the heating of the barreladjacent the gutters with the heating bands 134. The surface contaminantremoval is thus accomplished similarly to the surface removal operationin the foreline 93 described above.

As indicated above, motive fluid condensate will normally be foundpresent in very high concentrations in the lower portion of each gutter,particularly the gutters 150, 160 and 170, and their presence is in partthe reason for additional valves 157, 167 and 177 which may be providedin the surface draw off lines 151, 161 and 171. After removal of fluidcontaining light end contaminants from the surface portion of the gutter140, the drain line 146 may be opened to drain the lower portion of thegutter 140 into the collection vessel 152 (or admitted to the next lowergutter 150 as discussed above, for further enrichment processing). Thiswould be accomplished by opening only the line 153 to lower the pressurewithin the vessel 152 below that adjacent the gutter 140, then openingthe line 151 with the valve 157 closed, and opening the line 146. Whenthe fluid is collected in the vessel 152, the vessel could then bedrained through the line 154 for analysis by closing the lines 151 and153, opening the bleed line 155 and then opening the line 154. If thiscondensate comprises nearly pure motive fluid, it can be re-admitted tothe barrel and the boiler via the line 187. Condensates from the lowerportions of the gutters 150 and 160 can be drained similarly. The gutterdrain line 176 for the gutter 170 is of course provided with its owncollection vessel 182. During periods when the gutters are not beingdrawn from, the various condensates overflow successive gutters in awaterfall effect.

Since the pressure gradient within the diffusion pump barrel 91 is fromlowest at the top to highest at the bottom, opposite that of theforeline 93, the lightest of the heavy light end substances would beexpected to be found in the gutter 170, with the heaviest found in thegutters 140 and 150. However, due to the high concentration of motivefluid condensate in the lower gutters, it may be found advantageous touse only the uppermost gutter 140 for the collection of heavy light endsubstances. In fact, the lower three gutters, particularly the gutters160 and 170, would also contain very small concentrations of thelightest of the heavy end contaminants in the fluid spectrum, which mayescape the boiler skimming process. Heavy end substances of molecularweight and volatility very close to that of the basic motive fluid willbe carried along with the motive fluid through the chimney and nozzleassembly 90 into the diffusion pump barrel and will condense on thewalls of the barrel 91 even more readily than the motive fluid itself.Small concentrations of these substances may be found in fluid drainedfrom the bottom of the gutters 160 and 170. If fluid collected in thevessels 172 and 182 is found to be rich enough in the light heavy endcontaminants, these gutters may be drained periodically as part of aprogrammed pump barrel purification procedure.

The automatic operation of the diffusion pump barrel purification andseparation apparatus described above may be programmed according toresults of initial analysis. Variations in the type of pump fluid used,in the exact specifications of the pump boiler and foreline, and in theconditions within the vacuum chamber will dictate drastically differentresults in the collection gutters 140-170. After such analysis, theopening of the fluid withdrawal lines can be sequenced and durationtimed in order to provide maximum withdrawal of heavy light end andlight heavy end contaminants with minimum withdrawal of basic motivefluid. Of course, as with the operation of the foreline purificationmodifications, all fluid withdrawn into the collection vessels 142-182will contain a large amount of the motive fluid, but the system isdesigned to remove high concentrations of these contaminants as hasnever before been possible, so that pump performance can be increasedand ultimate attainable chamber vacuum can be additionally lowered byone or more decades.

As indicated above, the diffusion pump assembly 85 including boiler,foreline and barrel modificatons can be used as a "slave" pump to agroup of additional diffusion pumps serving the same vacuum system. Eachof the additional "master" pumps would have boiler and forelinemodifications only. The "slave-master" relationship between the pumpswould simply be one of boiler fluid exchange. For example, the slavepump shown in FIG. 5 would dispense portions of its highly purified pumpfluid from its boiler drain line 89 into the boilers of each of themaster pumps. Such fluid would be admitted to each master pump throughan auxiliary fluid adding line such as the line 187 shown in FIG. 5. Inturn, fluid from the boilers of the master pumps, very pure but not tothe extent of the fluid in the fully modified slave pump 85, would betaken from the boiler drain lines of each of the master pumps anddelivered through the auxiliary fluid add line 187 into the slave pump85 of FIG. 5. Such circulation between the pumps could be accomplishedslowly and continuously, but would preferably be done periodically, withthe slave pump 85 exchanging fluid with only one other pump at a time.

The fluid purification and separation apparatus of the above-describeddiffusion pump, including boiler, foreline and barrel modifications, canalso be applied, in total or in part, purely as means of fluidseparation in other disciplines apart from diffusion pumps and highvacuum production. For example, the surface heavy end skimmingmodifications of the invention can be employed in a fractionating columnfor the refining of petroleum. Surface skimming such as that describedherein can be employed at all evaporative surfaces in the refiningprocess--in the boiler and at each distillation plate in thefractionating column (not shown in the drawings). The employment of suchskimming modifications to refining equipment will very significantlyincrease throughput in the refining process by removal of heavy endfractions at all surfaces, thereby greatly reducing torpidity andproviding for continuous nearly 100% working evaporative surfaces. Inaddition, petroleum fractions withdrawn and collected through theskimming operations would comprise certain of the fractions desired tobe recovered. The fractions recovered in this way would of course varyfrom level to level in the column, with the heaviest heavy ends takenfrom the boiler and the lightest taken from the uppermost distillationplate. Efficiency, productivity and output of a refinery can thus beachieved at a minimum capital expenditure.

The extremely high purification and separation capability of theforeline and barrel modifications described above can be employed, intotal or in part, for the separation of the isotypes of uranium. Rawfeedstock uranium slurry would be heated in a boiler similar to theboiler 86 of the diffusion pump 85 shown in FIG. 5. In the foreline andbarrel gutters, means would preferably be provided for increasingcondensing area, such as baffles over the foreline gutters and stainlesssteel packing within the foreline and barrel gutters (not shown). Asmentioned above, such baffles slow pump throughput but increaseseparation capabilities. Gutter packing also increases separationcapabilities. The high vacuum and pressure gradient present in thediffusion pump barrel can be utilized to aid in theseparation/enrichment of the isotopes of uranium.

Separation of metals from one another and from impurities can also beeffected using the purification and separation apparatus describedhereinabove, in total or in part. The methods and apparatus of theinvention are particularly adaptable for the separation of metals in thelow and intermediate melting point range, such as those metals meltingbelow about 2500° F.

Various other embodiments and alterations to these preferred embodimentswill be apparent to those skilled in the art and may be made withoutdeparting from the spirit and scope of the following claims.

I claim:
 1. In a method for separating fractions of a liquid whichmethod includes the steps of heating a body of the liquid to atemperature sufficiently high that the liquid has a vapor pressure atleast substantially as high as the pressure to which the body of theliquid is subjected, condensing various fractions of the liquid atvarious levels above the surface of the heated body of liquid, andcollecting various liquid fractions at their respective condensationlevels, the improvement of withdrawing liquid at more than one levelfrom at least one liquid fraction collected at its respectivecondensation level and supplying heat to said various liquid fractionsto effect circulation and revaporization of said various liquidfractions.
 2. The method of claim 1 which further includes withdrawingfrom the heated body of liquid at least a portion of the upper surface.3. In a method for separating fractions of a liquid, which methodincludes the steps of heating a body of the liquid to a temperaturesufficiently high that the liquid has a vapor pressure at leastsubstantially as high as the pressure to which the body of the liquid issubjected, condensing various fractions of the liquid at various levelsabove the surface of the heated body of liquid, and collecting variousliquid fractions at their respective condensation levels, theimprovement comprising supplying heat to said various liquid fractionsand separately withdrawing portions of at least one collected liquidfraction from at least two vertically discrete levels of such collectedliquid fraction.
 4. In a method for separating fractions of a liquid,which method includes the steps of heating a body of the liquid to atemperature sufficiently high that the liquid has a vapor pressure atleast substantially as high as the pressure to which the body of theliquid is subjected thereby vaporizing the surface of said liquid andcondensing various fractions of the liquid at various levels above thesurface of the heated body of liquid to provide a volume of a collectedliquid fraction at each of said various levels, the improvement ofseparately withdrawing from at least one collected liquid fractionvolume a first upper portion from the surface of said volume and asecond portion from the remainder of said volume, said first portioncharacterized by the increased presence of liquid fractions of highermolecular weights than said second portion.
 5. In a method forseparating fractions of a liquid, which method includes the steps ofheating a body of the liquid to a temperature sufficiently high that theliquid has a vapor pressure at least substantially as high as thepressure to which the body of the liquid is subjected thereby vaporizingthe surface of said liquid and condensing various fractions of theliquid at various levels above the surface of the heated body of liquidto provide a volume of a collected liquid fraction at each of saidvarious levels, the improvement of separately withdrawing from eachcollected liquid fraction volume a first upper portion from the surfaceof said volume and a second portion from the remainder of said volumeand discharging such separately withdrawn portions to collectionvessels, said first portion characterized by the increased presence ofliquid fractions of higher molecular weights than said second portion.